Journal of Student Research 2014

Metallurgical Characterization

boundary diffusivity at elevated temperatures. This causes boron to diffuse away from the joint before reaching the brazing temperature thus preventing complete braze melting and bond formation. In addition, Ti, Al, Fe and Nb are added to most superalloys including Inconel-625 to strengthen the alloys and enhance their oxidation resistance by forming tenacious metallic oxides. Unfortunately, these non-wettable protective oxides hinder braze spreading and flow, restrict surface coverage, and impair the bond quality. Thus, brazing of superalloys involves issues that require careful metallurgical evaluation of joints brazed using different type of filler metals. The purpose of the study was to compare and contrast the microstructure, composition and microhardness of two vacuum brazed nickel-base superalloys, Inconel-625 and Waspaloy. These alloys were joined using five different commercial braze alloys. : two Ni-base amorphous brazes, two Ti containing Ag-base brazes, and a Cu-base active braze. The research had the dual objectives: (i) investigate how the process of brazing modifies the composition of a given braze and the metallurgical structure of the braze and the substrate in the vicinity of the joint via diffusion and reactions and, (ii) evaluate how different braze alloys modulate the metallurgical structure, composition and hardness in and around superalloy joints. An evaluation of the basic metallurgical characteristics of Inconel 625 and Waspaloy joined using multiple Ag, Cu and Ni-base braze alloys should permit screening of potentially promising systems for a detailed future investigation. Both Inconel-625 and Waspaloy retain high strength to elevated temperatures (870-980°C). They are used use under extreme heat and pressure environments such as those in gas turbines, combustors, and turbocharger rotors. Inconel-625 is an oxidation and corrosion-resistant austenitic Ni-based superalloy. Waspaloy is an age-hardenable, nickel-based superalloy with excellent strength properties. Generally, welding Ni-base superalloys is difficult due to cracking and microstructural segregation of alloying elements in the heat affected zone. Brazing such alloys works better than welding. Certain welding techniques can, however, be used with superalloys. Additionally, diffusion bonding [Ahmad et al, 2008], diffusion brazing [Ojo et al, 2004; Laux et al, 2010], transient liquid phase bonding [Egbewande et al, 2008; Ojo et al, 2004], and liquid infiltration

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